1. Field of the Invention
The delignification of wood chips with oxygen and a cooking liquor.
2. Description of the Prior Art
An encomical nonpolluting pulping system would be an oxygen delignification system in which chips would be pulped in one cooking cycle, a one-stage system, in a relatively short time span. Unfortunately, a one-stage cook in an alkali-oxygen system results in a chip having an overcooked exterior and an undercooked interior creating a low yield in the weight of pulp that can be obtained from a given weight of wood chips. Theoretically, the yield from this process should be greater than the comparable yield from a kraft pulping process.
The reason for the overdelignification of the outer portion of a wood chip and the springwood within the chip and the underdelignification of the inner portion of the wood chip and the summerwood within the chip is that oxygen is a poorly soluble gas, and transfers through the cooking liquor and uniformly into the wood chips only with difficulty.
Samuelson, on page 1 of U.S. Pat. No. 3,764,464 issued Oct. 9, 1973, describes the problem and some methods that have been used in an attempt to overcome them. There have been several approaches to obtaining a uniform diffusion of oxygen through the wood chip. Makkonen et al, in "Preliminary Studies of Peroxide and Oxygen Pretreatment in Alkaline Pulping," a paper presented to the 25th TAPPI Alkaline Pulping Conference of 1971, and the article by Palenius, "Soda-Oxygen Cooking" Revue A.T.I.P., 28, 4 pp. 201-206 (1974), indicate there are three major methods being used.
The first was a pulping stage followed by an oxidative treatment as in present bleaching practice. This is the approach being used in the many patents and articles describing the oxygen bleaching process, or the two-stage oxygen pulping process. In this approach, the chip is fiberized prior to treatment with oxygen so there is no chip interior or exterior, only fibers. The chips normally are pulped using a standard process such as kraft or soda, and the chips are thereafter defiberized as by being blown from the high pressure digester into a low pressure blow tank or by a mechanical refining step. Illustrative of the many patents and abstracts describing these processes are Robert et al U.S. Pat. No. 3,384,533 issued May 21, 1968, and Worster et al U.S. Pat. No. 3,691,008 issued Sept. 12, 1972.
The second method is a combined oxidative pulping system. These are the one-stage systems in which wood chips are pulped in the presence of oxygen and a cooking liquor. In this system, the conditions are manipulated in an attempt to obtain the diffusion of the oxygen uniformly through the chip. Typically, these manipulations take the form of low cooking temperatures and very long, and therefore expensive, cooking times to obtain a mild cook throughout the chip, and diffusion throughout the chip; a continuous infusion of chemicals and oxygen into the cooking liquor in order to maintain the cooking liquor at its initial conditions; and, most recently, mechanically removing the cooked fibers from the chip and the cooking liquor to create continually a new outer chip surface. Typical patents and articles describing the first two approaches are: Chang et al, "Delignification of High-Yield Pulp with Oxygen and Alkali--Southern Pine Kraft Pulp," TAPPI, January 1973, Vol. 56, No. 1, pp. 97-100; Shchyegolyev et al, "The Influence of Molecular Oxygen Upon the Process of Delignification," Bumazhnaya Promushlyennos, 1966, No. 5, 6-7; Samuelson, U.S. Pat. No. 3,701,712, issued Oct. 31, 1972; Croon et al, U.S. Pat. No. 3,652,388, issued Mar. 28, 1972; Samuelson et al, U.S. Pat. No. 3,769,152, issued Oct. 30, 1973; Harris, U.S. Pat. No. 2,673,148, issued Mar. 23, 1954; Nowakowski, "The Role of Oxygen at the Alkaline Digestion of Softwood Under Pressure," Zeszyty Problemowe Postepow Nauk Rolniczych No. 52 (1965), pp. 107-126; Minor et al, "Factors Influencing the Properties of Oxygen Pulps from Softwood Chips," TAPPI, Vol. 57, No. 5, May 1974, pp. 120-122 and paper presented at 1973 TAPPI Alkaline Pulping Conference, pp. 52-56; Solano, "A Study of Oxygen--Soda Pulping on Eucalyptus Globulus," Masters thesis, June 1972, Syracuse University; and Worster et al, Canadian Pat. No. 895,757, issued Mar. 21, 1972. The above patents and articles indicates no general trend in methods of treating wood chips with oxygen in one stage. They seem to indicate different results using similar conditions. For example, several advise that high initial concentrations of oxygen are detrimental to the pulping process, but one indicates that with hardwood chips good results were obtained with high initial chemical and oxygen concentrations.
The third approach, the use of mechanical means to remove the outer surface of the chip as it reacts, is described in Samuelson, German Patent Disclosure No. 2,441,440, disclosed Mar. 27, 1975, entitled "Process for Pulping of Wood." Various means for removing the outer fibers from the chips and removing these fibers from the pulping liquor are described.
A third method is an oxidative pretreatment followed by pulping. This is described in the above Makkonen and Palenius articles and is clarified by Karna, Johansen and Sarkanen. This method uses both a cooking liquor and oxygen at the same time, even though it is called an oxygen-alkali system. It has the same diffusion problems as the first approach. Its purpose is to stabilize the carbohydrates. Another approach described in Richter, U.S. Pat. No. 1,831,032, issued Mar. 30, 1928, is to cook the chips in water with an oxygen over-pressure.
Sunds British Pat. No. 1,360,839 discloses the pulp falling through a gas and coming to rest in a column having a consistency of around 25-30%. The pulp had been mixed with 3-5% sodium hydroxide prior to the experiments. It appears that the column of pulp sits in oxygen and the consistency level is required so that combustion does not occur. Mitchell et al, U.S. Pat. No. 2,811,518, also requires that the pulp treated in the alkaline-oxygen stage be loose, fluffy and not be immersed in liquor. It is noted in Col. 3, lines 26-36.
All these methods and approaches contact the chips with the cooking liquor before or at the time of application of oxygen.